340 CHITONID^. 



CYMATOCHiTON, Dall. Leptochitoii with the valves thrown for- 

 ward. Differs from Probolseum in the valves being transverse 

 instead of squared, and in the terminal valves being regular 

 instead of waved. G. Loftiisianus, King (Ixxxv, 68, 69). 



Recent Forms. 



LEPTOCHiTON, Gray. (Lepidopleurus, Risso. Craspedochilus, 

 and Boreochiton, Sars.) Insertion-laminae none ; girdle or zone 

 minutely sandy ; sinus smooth, not laminated. Branchiae short. 

 Distr. — 10 sp. Northern Seas, Kerguelen's Isl. Type, L. asellus, 

 fjowe. L. cancellatuSj Sowb. (Ixxxv, 71^. Only 2 out of the 

 25 species cited by Adams really belong here. 



DESHAYESiELLA, Carpenter. Yalves curved forwards ; mucro 

 planate ; zone or girdle spiculose ; insertion-plates none ; sutural 

 laminae triangular, raised. C. (Leptochiton) curvatus^ Cpr. 



HANLEYiA, Gray. Anterior valve with an unslit insertion- 

 plate ; other valves destitute of the plates. Distr Mostly 



northern ; one from deep waters. Gulf of Mexico. G. debilis, 

 Gray. 



HEMIARTHRUM, Carpenter. Insertion-plates present on all the 

 valves, but entire without slits. H. setulosum, Carpenter. 



MiCROPLAX, H. Adams. Resembling Chitonellus externally ; 

 submerged laminae unslit, entire, fused in an undistinguishable 

 manner with the parts which usually constitute the sutural 

 laminae. M. Grayi, Ad. and Ang. 



B. Lschnoidea. 

 Insertion-plates sharp, smooth, fissured ; with eaves. 



* No pores on girdle. 



TRACHYDERMON, Cpr. (Lepldopleurus sp., Auct. Craspedo- 

 chilus, Sars.) Insertion-laminae acute, smooth ; valves within 

 and without exactly like Ischnochiton ; zone or girdle not porif- 

 erous, granulated by very minute scales. Branchiae short. This 

 name was originally proposed as a subgenus of Ischnochiton to 

 include Gray's second section, "mantle-scales minute, granular." 

 In all other conchological characters the group accords with that 

 genus, but the animal difl'ers in having the gills either entirely 

 posterior or reaching forward from the tail only to about the 

 middle of the foot, while in Ischnochiton and Chiton they travel 

 to its anterior extremity. These characters indicate a transition 

 between the Ischnoid and Leptoid Chitons by means of Trachy- 

 dermon and Tonicella. Distr. — Chiefly northern. G. ruber, 

 Lowe (Ixxxv, 72). 



TRACHYRADsiA, Cpr. Central valve doubly or many-cleft. G. 

 fulget7-um, Reeve. 



CALLOCHiTON, Gray. Laminae broken up into very numerous 

 teeth rising out of spongy eaves, and having a tendency to 



